1. Field of the Invention
This invention relates to electrical communications and more particularly to error prevention systems. In the preferred application, this invention relates to dynamic information storage and retrieval and more particularly to error prevention in connection with automatic control of track centering of a magnetic recording head.
2. Description of the Prior Art
In the field of magnetic recording, in the past where there have been separate magnetic recording heads for data and track sensing for positioning of a magnetic recording head, it has been necessary to provide protection against magnetic and electrical cross-talk between channels. In order to prevent such cross-talk, wires have been shielded and heads have been separated by magnetic shielding material. In fact, in many cases the head used to control tracking of the servo mechanism is placed far away from the head which operates to read and/or record data, a situation which introduces positioning errors due to intermittent mechanical factors such as thermal expansion and contraction or vibration of the two heads. In some cases, the track following head must be located adjacent a separate disk, which increases the probability of error even more.
Previous work of P. A. Franaszek, who is a coinventor herein, is described in U.S. Pat. No. 3,840,854. The patent teaches a technique for modifying the D.C. component of a binary code by introduction of ones into strings of zeroes and introduction of zeroes into strings of ones so that there is substantial elimination of the D.C. component of a signal.
A publication of Furtney et al entitled "DC Balanced Encoded Recording," IBM Technical Disclosure Bulletin, Vol. 18, No. 1, June 1975, pp 53-54 shows a code selection circuit which is used to provide a D.C. balance of a recording signal similar to that in U.S. Pat. No. 3,840,854.
J. H. Bleher wrote a publication entitled "Data Code Servo," IBM Technical Disclosure Bulletin, Vol. 18, No. 6, Nov. 1975, p. 1917 which shows adjacent track heads on each side of a data recording head, with alternating codes upon adjacent tracks so that with the two servo heads straddling adjacent tracks, the low pass result of the signals received is used to control a servo.
In a publication by L. F. Shew entitled "Track Servo System," IBM Technical Disclosure Bulletin, Vol. 16, No. 7, Dec. 1973, pp 2225- 6 there is described the need to place the servo track immediately next to the data track in order to overcome errors caused by differences in temperature which lead to large positioning errors. The publication proposes that the data on the track or an adjacent track plus very small sample servo sectors should be used to control head position. This takes a substantial fraction of the data track and it requires a rather sophisticated servo control system responding to intermittent signals which can be overcome by sharing a single servo track with a number of other tracks.
Accordingly, it is desirable to reduce the chance of cross-talk between the servo track and the data track, respective heads and/or respective leads.
Another object of this invention is to provide means for generating data signals having a spectral null at the frequency of the servo signal track.